Conventionally, retroreflective sheeting capable of reflecting incident light back toward the light source have been well known, and such sheeting is widely used in the above-described fields of application due to its retroreflectivity. Among others, retroreflective sheeting utilizing the retroreflection principle of prisms, such as cube-corner retroreflective sheeting, has markedly higher optical retroreflection efficiency than conventional retroreflective sheeting using micro glass beads, and the range of its use is expanding every year because of its excellent retroreflection performance.
On the basis of their principle of reflection, cube-corner retroreflective elements exhibit high retroreflectivity, so long as the angle between the optical axis [i.e., an axis lying at an equal distance from three mutually perpendicular faces constituting the prismatic reflective elements (sometimes referred to simply as "prismatic element")] of a prismatic element and incident light (i.e., the entrance angle) is narrow. However, cube-corner retroreflective elements have the disadvantage that, as entrance angle increases, their retroreflection efficiency is reduced. Moreover, when rays of light are incident on a lateral face at an angle greater than the critical angle satisfying the conditions for total internal reflection which are determined according to the ratio of the refractive index of the transparent medium constituting the retroreflective element to the refractive index of air, most of them do not undergo total reflection at the interfaces of the prismatic element but pass to the back side of the prism. Thus, they have the additional disadvantage that the range of entrance angle which permits retroreflection is limited.
In order to overcome these disadvantages, various attempts have been made to improve the method of making a mold used for the formation of prisms. Some typical methods of making a prism mold which have been proposed in prior arts are described below.
(1) Bundled pin method (U.S. Pat. Nos. 1,591,572, 3,922,065 and 2,029,375):
This is a method in which a large number of metallic pins having a prism formed at the tip thereof are bundled to form an array of prisms. This method is characterized in that the design of the prism formed at the tip of each pin may be arbitrarily modified and is suitable for the production of relatively large prisms. However, it is not practical when the formation of, for example, more than 2,000 microprisms per square centimeter is required as dictated by the object of the present invention.
(2) Plate method (U.S. Pat. Nos. 1,591,572, 3,069,721 and 4,073,568):
This is a method of making a microprism mold of the hexagonal prism type which comprises stacking a plurality of flat sheets having two mutually parallel major surfaces, cutting therein V-shaped grooves in a direction perpendicular to the major surfaces and at a fixed pitch to form a series of successive roof-shaped projections having a vertical angle of about 90.degree., and then shifting the flat sheets so that the vertices of the roof-shaped projections formed on each flat sheet meet the bottoms of the V-shaped grooves formed on an adjacent flat sheet. This method is characterized by a relatively better design freedom, though it is lower than that of the bundled pin method. This method can improve the poor productivity in the fabrication of a prism mold which constitutes a disadvantage of the above-described bundled pin method. However, this method has the disadvantage that, in case of forming microprisms, the insufficient strength of flat sheets may cause them to become distorted during the cutting of V-shaped grooves, and has hence been used for the production of relatively large prisms.
(3) Triangular prism method (U.S. Pat. Nos. 3,712,706 and 2,380,447):
This is a method in which V-shaped grooves extending in three different directions are cut in a surface of a flat plate made of a metal or the like to form an array of prisms thereon. This method has frequently been employed for the production of conventional retroreflective sheeting using prismatic elements. The reasons for this are that it is relatively easy to form microprisms by cutting and that thin retroreflective sheeting may be obtained because it is possible to form an array in which the bases of the formed triangular prisms are arranged in a common plane. However, this method has the disadvantage that the prism shape which can be employed is limited to triangular prisms capable of being formed by V-groove cutting and its degree of design freedom is low.
Next, the properties desired for retroreflective sheeting and the problems involved in cube-corner retroreflective sheeting using prismatic elements are described below.
Generally, the basic properties desired for retroreflective sheeting include high reflectivity (i.e., the highness of reflective brightness as represented by the reflective brightness of light incident on the sheeting from a direction perpendicular to the surface thereof) and angularity. Moreover, angularity involves the following three considerations.
A first consideration relating to angularity is observation angularity. Where retroreflective sheeting is used, for example, in various signs such as traffic signs, the location of the viewer is usually different from that of the light source. Accordingly, more intense light must reach the viewer positioned away from the axis of the entrance light. Therefore it is required that the reduction in reflectivity should be limited even at the location of wide observation angle to the axis of the incident light.
A second consideration relating to angularity is entrance angularity. For example, as an automobile is approaching closer to a traffic sign, the entrance angle of light emitted by the headlights of the automobile to the sign increases gradually, and the intensity of the light reaching the driver being the viewer decreases correspondingly. In order to cause the sign to retain sufficiently high reflectivity even when the river approaches to the sign, excellent entrance angularity is required.
A third consideration relating to angularity is rotational angularity. A phenomenon peculiar to prismatic elements is such that retroreflectivity varies according to the direction from which light is incident on the retroreflective sheeting. Consequently, retroreflective sheeting involves a troublesome problem which is that the sheeting should be applied in control of the direction of the sheeting when applying it to signs. Micro glass bead type retroreflective sheeting does not involve this problem because the reflective elements have the form of a body of revolution.
Usually, prismatic retroreflective sheeting is characterized in that the right retroreflectivity thereof is two to three times higher than that of bead type retroreflective sheeting, but it is generally said to have poor angularity. For this reason, in order to satisfy trihedral reflection requirements based on the retroreflection principle of cube corner, the angle of incidence must be relatively close to 0.degree., i.e., light must be incident on the retroreflective sheeting from a direction substantially perpendicular to the surface thereof. If the angle of incidence becomes greater, the light may fail to reach a second or third reflective lateral face and escape out of the prism, resulting in a reduction in retroreflection efficiency. Moreover, if the angle of incidence exceeds a certain limit, the conditions for total internal reflection are not satisfied, so that the incident light passes to the back side of the prism.
In order to overcome the above-described disadvantages, there has generally been employed a method wherein the optical axes of the prismatic elements, which are conventionally oriented so as to be perpendicular to the surface of the retroreflective sheeting, are slightly tilted in various directions to increase their retroreflection efficiency toward the tilting directions.
For example, in the triangular prismatic method, it has been proposed to vary slightly the angle of intersection of V-shaped grooves which generally intersect with each other at an angle of 60.degree. (U.S. Pat. Nos. 4,588,258 and 4,775,219). Since the optical axes tilted by this method are obtained only in the form of pairs of prisms facing in opposite directions forming an angle of 180.degree., an improvement in angularity can be achieved in the directions of tilting of the optical axes, but no improvement is achieved in other directions. Moreover, no improvement in rotational angularity is achieved.
In order to overcome the above-described disadvantage that the conditions for total internal reflection are not satisfied when the entrance angle exceeds a certain limit, it has been proposed to coat the reflective lateral faces with a metal film or the like and thereby cause specular reflection (U.S. Pat. Nos. 3,712,706 and 2,380,447). However, this method has the disadvantage that the resulting sheeting has a dark appearance and the metal film is susceptible to moisture or the like.
Rotational angularity poses a serious problem especially in the case of triangular prisms. In order to improve rotational angularity, there is known a method in which the prism array surface is divided into plural sections of certain size and directions of sections are changed with respect to each other (see U.S. Pat. No. 4,243,618). In this method, the rotation angle of light incident on the prisms differs from section to section, and the reflectivity varies correspondingly. When viewed from a long distance, a whole reflectivity is leveled to give uniform rotational angularity. However, the sections of the prism array surface can be rather clearly seen from the front side of the retroreflective sheeting, and hence have the disadvantage that the appearance of the sheeting is reduced in attractiveness.
Moreover, in prism molds which can be applied to the fields of application of the present invention and can be used for the production of relatively thin and flexible retroreflective sheeting, it is desirable that the prismatic elements have a minute size, for example, of 500 .mu.m or less. However, it is difficult to produce such reflective sheeting according to the above-described bundled pin method and plate method. The triangular prism method permits the formation of minute prisms, but it is difficult to carry out the design of prisms having excellent angularity as is another object of the present invention.
In the aforementioned U.S. Pat. No. 1,591,572 to Stimson, there is described a method for making a prism mold by using glass rods or sheets having one end formed into the shape of a prism or prisms. However, the flat sheets used in the method described therein has such low strength that this method is not suitable for the formation of microprisms desired in thin retroreflective sheeting as dictated by an object of the present invention.
It is described in the aforementioned U.S. Pat. No. 3,069,721 to Arni et al. that optically flat metal faces can be obtained by cutting flat metal sheets with a diamond cutter and that prism sheeting can be formed by using prism-forming metal sheets obtained by this method. However, neither description nor suggestion is given therein as to the formation of a microprism master mold having excellent characteristics by using thin flat sheets made of a synthetic resin.
It is an object of the present invention to provide a method of making a microprism master mold which is suitable for use in the production of cube-corner retroreflectors, in particular in the form of thin retroreflective sheeting, and permits the formation of hexagonal prism type microprisms having both high reflectivity and excellent angularity, by focusing attention on the above-described plate method and overcoming its disadvantages while maintaining its advantages.